1. Field of the Invention
The present invention relates to an active vibratory noise control apparatus for canceling vibratory noises produced by a vibratory noise source by means of vibratory noise canceling sounds that are opposite in phase to the vibratory noise, and more particularly to an active vibratory noise control apparatus for reducing vibratory noises produced within a passenger compartment of a vehicle by a vibratory noise source such as the vehicle engine.
2. Description of the Related Art
Conventional active vibratory noise control apparatus operate by detecting noise in the passenger compartment of a vehicle by means of a microphone disposed centrally over the front seats near the position of an ear of a passenger, then generating a signal that is opposite in phase to an output signal produced by the microphone based on the noise, and outputting canceling sounds based on the generated signal into the passenger compartment from two speakers that are mounted respectively in the left and right doors alongside of the front seats, for thereby reducing the noise at the microphone (see Japanese Laid-Open Patent Publication No. 2003-47097).
As shown in FIG. 3 of the accompanying drawings, when the frequency of the sound heard by the ear of the passenger in the passenger compartment increases nearly to 140 Hz, for example, one-half of the wavelength of the canceling sound becomes nearly (L3-L4), representing the difference between a distance L3 from a speaker 28b on the right side (left side in FIG. 3) of the vehicle 12, as viewed from the passenger to an ear position 80 of the passenger, and a distance L4 from a speaker 28a on the left side (right side in FIG. 3) of the vehicle 12 as viewed from the passenger to the ear position 80. At the ear position 80, therefore, the canceling sounds from the speakers 28a and 28b interfere with each other.
According to Japanese Laid-Open Patent Publication No. 2003-47097, a phase shifter generates signals by shifting the central frequency of the phase rotation of the signal in an opposite phase, and supplies the generated signals to the respective speakers. In this manner, even when the frequency of the sound in the passenger compartment becomes higher, the canceling sounds from the left and right speakers are prevented from interfering with each other.
However, since the phase shifter is added to the apparatus for reducing noise in the passenger compartment, and the opposite phase signal is rotated in phase by means of the phase shifter, the active vibratory noise control apparatus has a complex configuration and is high in cost.